Poster Presentation 31st Lorne Cancer Conference 2019

Inhibiting PRMT5 as a novel therapeutic approach for the treatment of melanoma (#186)

Prerana Ghosh 1 2 , Shatha AbuHammad 1 2 , Laura Kirby 2 , Teresa Ward 2 , Grant A McArthur 1 2 , Karen E Sheppard 1 2
  1. The University of Melbourne, Parkville, Victoria, Australia
  2. Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia

Melanoma is the deadliest form of skin cancer. In majority of melanomas, the tumour suppressors, RB1 and p53 are inactivated. Phosphorylation and inactivation of RB1 is due to increased CDK4 activity, which is a function of activating mutations in RAS/RAF/MEK/ERK (MAPK) signalling and loss of p16, the negative regulator of CDK41,2. Most melanomas harbour wild-type p53 (p53WT) that is inactivated via high MDM4 expression3,4. Recent studies in the lab have revealed that palbociclib, a CDK4 inhibitor, not only reactivated RB1 but also activated p53.  Activation of p53 was via palbociclib mediated inhibition of protein arginine methyltransferase 5 (PRMT5) which in turn decreased MDM4 levels. Additionally, combining palbociclib with a PRMT5 inhibitor potently inhibited cell proliferation. Considering CDK4 is activated by the MAPK pathway we hypothesized that inhibition of the MAPK pathway will decrease PRMT5 activity and activate p53 and as such the combination of a MAPK inhibitor with a PRMT5 inhibitor would lead to a robust response. This study aimed to examine the effects of inhibition of mutant BRAF (BRAFV600E) with PLX-4720 on PRMT5 activity, MDM4 expression and p53 activation. Additionally, the response to the combination of PLX-4720 and GSK3326595 (PRMT5 inhibitor) on A375 human melanoma cell proliferation was assessed. Western immunoblot and qRT-PCR analysis demonstrated that inhibition of BRAFV600E decreased PRMT5 activity and MDM4 expression while promoting p53 activation. Cell proliferation assays revealed that co-inhibition of PRMT5 and BRAFV600E potently inhibited cell proliferation and was more efficacious than single agent therapy. Considering MDM4 is frequently overexpressed in melanoma and one of the mechanisms behind suppression of p53 activity, this combination serves as a promising therapeutic approach that can reactivate the p53 pathway leading to inhibition of cell proliferation.

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